Tall fescue, grown on over 35 million acres, is the most widely spread pasture grass in humid areas of the eastern U.S.A. and, to a limited extent, in the northwestern U.S.A. It is also commonly used for vegetative cover on highway banks, parks, playgrounds, home lawns, and waterways.
There are many reasons for its popularity: ease and wide range of establishment, wide range of adaptation, long grazing season, tolerance to abuse, pest resistance, good seed production, and excellent appearance when used for non-forage purposes. Tolerance of tall fescue to adverse climate, soil, and management conditions has aroused new interest and stimulated breeding and selection programs in countries such as France, Japan, Australia, New Zealand, and the USSR.
With increased use of tall fescue in pastures, beginning in the 1940's, there soon were disturbing reports of poor animal performance and visible disorders. This was puzzling since digestible dry matter, crude protein, amino acid, and mineral content suggested that well managed tall fescue should give good animal performance. However, in grazing studies, beef steer gains were low, usually only about 1 lb/day for the season. Gains on tall fescue were substantially lower than on orchardgrass. With beef cattle, calf gains and cow conception rates were substantially lower on tall fescue than on tall fescue-clover.
There are three principle symptoms of fescue toxicosis. The most dramatic visible symptom occurring on cattle grazing tall fescue is "fescue foot," a gangrenous condition of feet and/or tails. This syndrome appears to be related to lower ambient temperature and is much more widespread in northern than in southern parts of the tall fescue-growing region.
The second syndrome of cattle grazing tall fescue is bovine fat necrosis, which is characterized by hard fat masses and abdominal fat tissue deposits, often in adipose tissue surrounding intestines, causing poor digestion and calving problems. Originally associated with heavy applications of broiler litter to tall fescue pastures, it was later found the syndrome could occur with high nitrogen fertilizer.
The third and most common syndrome associated with tall fescue is "summer slump" or summer fescue toxicosis because of accentuated poor animal appearance and performance in summer. It is characterized by poor animal gains, intolerance to heat, excessive salivation, rough hair coat, elevated body temperature, nervousness, dramatically reduced weaning weights, lower milk production of both beef and dairy cows, and reduced pregnancy rate. In contrast to fescue foot and fat necrosis syndromes, summer fescue toxicosis is common and widespread throughout tall fescue-growing regions.
In 1973, J. D. Robbins, a chemist at the USDA Russell Research Center in Athens, Georgia, fungal physiologist C. W. Bacon and medicinal organic chemist J. K. Porter hypothesized that a fungus might be involved in the toxic syndrome. This hypothesis was based on work in New Zealand which showed tall fescue was subject to infection by an endophyte. Sampling of pastures in Georgia and four other states led them to postulate in Appl. Environ. Microbiol. 35, 576-581 (1977), that the fungal endophyte Epichloe typhina was associated with fescue toxicosis in cattle. The endophyte was later reclassified as Acremonium coenophialum. They predicted the endophyte was seed transmitted and would die if seed were stored for 1 to 2 years.
Subsequent grazing trials did not prove a cause and effect relationship, but did confirm that the fungal endophyte was associated with fescue toxicosis and that excellent animal performance could be achieved on low-endophyte tall fescue.
Since the early studies that associated the endophyte with reduced animal performance, considerable research has been devoted to confirming and documenting the extent of detrimental effects of endophyte-infected tall fescue. Generally, steer ADG has been increased from 30 to over 100% by shifting from high- to low- endophyte pastures. Gain per acre has been increased to a lesser extent, probably a result of lower intake and resulting higher carrying capacity of high-endophyte pastures. Intake is 10 to 50% higher on low- than on high-endophyte hay or seed. Results of studies appear to indicate a linear relationship between endophyte level and reduction in steer gains. Stuedemann et al., J. Anim. Sci., 63 (Suppl.1), 290-291 (1986) observed a significant linear relationship between ADG and average percent endophyte, indicating that for each 10% increase in endophyte frequency, there was a 0.12 lb depression in ADG.
Cattle grazing toxic tall fescue have a tendency to wallow in mud, particularly during hot times of the day, as well as spend much less time grazing than cattle on other grasses. When moved from high- to low-endophyte tall fescue, steers grazed an amount of time similar to those remaining on the high-endophyte fescue for at least 26 days following exchange, indicating that grazing high-endophyte tall fescue has a residual effect.
Although intake reduction could account for much of the difference in animal performance on low- and high-endophyte tall fescue, the decreased grazing behavior even after the cattle are removed from the high endophyte fescue and systemic effects, including fescue foot and bovine fat necrosis, appeared to be a result of a toxic substance(s) present in the endophyte infested fescue. Several clinical signs of tall fescue toxicosis including increased body temperature, reduced performance, and rough hair coat could be caused by heat stress or administration of a pyrogenic substance. Inhibition of rumen microflora activity, particularly cellulolytic activity, by perloline and loline alkaloids, suggested alkaloids might be responsible.
A great deal of research has now been done on various alkaloids present in toxic fescue. Although a causal relationship with fescue toxicosis has not been demonstrated, steers grazing G1-307 (an experimental, high-endophyte line with low perloline concentration and high N-acetyl plus N-formyl loline alkaloid concentration) exhibit the greatest signs of summer fescue toxicosis and have the lowest serum cholesterol concentrations, as reported by Stuedemann et al., in Am. J. Vet. Res. 46, 1990-1995 (1985). It therefore appears that toxic tall fescue influences lipid metabolism, possibly due to action by alkaloid(s) present in the grass.
Few controlled grazing studies have been conducted with sheep to determine the effect of endophyte infection on animal performance. A number of studies have utilized sheep as models in controlled metabolism or physiology studies. Generally, these studies indicate responses similar to those found with cattle, though there may be magnitude differences.
Controlled studies have not been conducted with horses, but tall fescue, presumably endophyte-infected, has been associated with reproductive and agalactia problems in mares. Monroe et al., J. Anim. Sci. (Suppl.1): 50 (1987), studied mares grazing endophyte-infected and endophyte-free tall fescue with and without selenium. Mares grazing infected fescue showed a greater incidence of agalactia (88%) and retained placentas (75%). There have been other case reports of high foal mortality, low conception rates and agalactia in mares grazing on fescue infected with the endophyte.
At the present time, there are few options for preventing or treating fescue toxicosis. A recent estimate blames fescue toxicosis for losses by the livestock industry of between 200 million and one billion dollars per year. With high levels of endophyte infection, pasture replacement may be the preferred method for preventing fescue toxicosis. It is more difficult to assess the cost effectiveness of renovating pasture having intermediate levels of infection by the endophyte. It would be preferable to have a practical means for treating animals grazing or ingesting infected fescue with the flexibility of addressing the problem on an individual basis.
It is therefore an object of the present invention to provide a method for treating fescue toxicosis in grazing animals, especially cattle, sheep and horses.
It is another object of the present invention to provide a commercially useful drug and means for administering the drug which prevents fescue toxicosis in animals grazing on infected pastures or ingesting harvested infected fescue.
It is a further object of the present invention to provide a method and compositions for preventing fescue toxicosis in grazing animals which do not have serious side effects or create problems with the handling or end utilization of the animals.